1. Field of the Invention
This invention relates to machine-readable color symbology for encoding data, such as color barcodes, and in particular, it relates to a method for enhancing the data capacity and readability of color gradation barcodes for a closed loop system.
2. Description of the Related Art
Barcodes are a form of machine-readable symbology for recording digital information. One-dimensional and two-dimensional black and white barcodes have been widely used. To increase data capacity, two dimensional color barcodes have been proposed, which use color in addition to the intensity of light reflection to represent information. For example, U.S. Pat. No. 6,070,805 describes a distortion-resistant color barcode symbology that uses two sets of colors (red, green, and blue, and cyan, magenta and yellow). In another example, U.S. Patent Application Publication 2005/0284944 A1 describes using six colors (red, orange, blue, green, yellow and pink) in a color barcode.
The rendering and detection of color are influenced by many factors. To apply a color barcode on a recoding medium such as paper, transparency, plastic sheet, fabric, etc., a color printer is used to print the barcode. Color printers typically use toning materials, such as color inks and toner particles, of the subtractive colors, i.e., cyan (C), magenta (M) and yellow (Y). Together with black (K), they are often referred to as CMYK colors. Colors printed by a printer depend to a certain extent on the characteristics of the toning material sets used by the printer. Color barcode may also be displayed on a display device such as CRT, LCD display, etc. Color displays typically use display pixels having additive colors, i.e., red (R), green (G) and blue (B). To read a color barcode, a color scanner or other detector device is used to scan the barcode. A color scanner or detector typically uses a set of color filters (e.g. RGB filters) to detect the color of the received light. In addition, both a printer or display device and a scanner use software to manage color data. For example, different software may use different color profiles for converting between RGB and CMYK colors. Thus, the same digital color data (e.g. as described by the RGB values of a color) often results in different printed colors when printed by different printers. Sometimes the printed colors also depend on the characteristics of the recording medium. Likewise, the same printed color often results in different digital color data when scanned by different scanners. The same is true for displayed colors. Further, the same digital color data often result in different colors when displayed on a display device and when printed. As a result of these factors, it is often difficult to accurately produce, measure and reproduce color.
These hardware factors limit the data capacity and readability of color barcodes. For example, even if a printer can nominally print 8-bit color data per color (i.e. 256 color gradation levels), a scanner usually cannot accurately measure 256 shades of a color printed by a printer.